Method of improving electrical signal transmission in wells



Feb. 16, 1965 H. BRANDT METHOD OF IMPROVING ELECTRICAL SIGNAL TRANSMISSION IN WELLS 5 Sheets-Sheet 1 Filed July 12, 1962 u... U .u w. a .W.W @xm INVENTOR HARRY BRANDT Feb. 16, 1965 H. BRANDT METHOD OF IMPROVING ELECTRICAL SIGNAL TRANSMISSION IN WELLS 5 Sheets-Sheet 2 Filed July 12, 1962 .3: 6922 w d oml. od E m doom k o "I u m". od-"TR M d m h od r m u wl. o ral 9 .LIOA "lVlLNI-ilOd aaddoa INVENTOR HARRY BRANDT BY 44% arm A? LAM/)- ATTORNEYS Feb. 16, 1965 H. BRANDT 3,170,137

METHOD OF IMPROVING ELECTRICAL SIGNAL TRANSMISSION IN WELLS Filed July 12, 1962 5 Sheets-Sheet 3 CURVE 2 1 1 1o 15 2o 25 APPLIED CURRENT, ma FIG 6 o o o o o o O o o o In 0 In SWHO QQNVISISEB' INVENTOR HARRY BRANDT Feb. 16, 1965 H. BRANDT METHOD OF IMPROVING ELECTRICAL SIGNAL TRANSMISSION IN WELLS 5 Sheets-Sheet 4 Filed July 12, 1962 h U L zwmmau omjmnz mm on mm om 2 OOON SWHO aouvislsau INVENTOR HARRY BRANDT Feb. 16, 1965 H. BRANDT METHOD OF IMPROVING ELECTRICAL SIGNAL TRANSMISSION IN WELLS 5 Sheets-Sheet 5 Filed July 12, 1962 10 APPLIED CURRENT, ma

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Q m N n 9 R G Cl 0 F E L P P A 5 O O O O o 0 0 Q 0 o O Q Q 0 o o o o O O o o o 0 O United States Patent 3,170,137 METHOD OF IMPROVING ELECTRICAL SIGNAL TRANSMISSION IN WELLS Harry Brandt, Whittier, Caliii, assignor to California Research Corporation, San Francisco, Calif, a corporation of Delaware Filed July 12, 1962, Ser. No. 299,453 14 Claims. (Cl. 340-18) This invention relates to the transmission of electrical signals in wells, and more particularly this invention relates to methods of improving the transmission of electric signals along a segmented drill string located in a fluid-filled borehole.

It is often desirable to send or receive electrical signals between the surface and downhole when drilling a borehole or when operating a well. Transducers to make measurements at bottom hole conditions and surface instruments for receiving and displaying signals from the transducers are known in the art. For example, a forcemeasuring device producing an electrical signal proportional to the force on a drill bit may be attached to the bit and an electrical signal transmitted to the surface to give force information to an operator. Devices producing signals proportional to temperature and resistivity are also frequently used. In a like manner it is desirable to send signals from the surface to instruments located in the drill collar. However, the means of transmitting the signals between downhole and the surface presents serious difiiculty.

As is known in the art of well drilling, a borehole is advanced into the earth by the action of a drill bit against the drilling face. The drill bit is attached to a drill collar of a drill string and is supplied with rotational or reciprocating motion by the drill string and to some extent is weighted by the drill collar. The drill string is made up of a number of drill pipes mechanically coupled together. The drill pipes are usually-threaded together at the drilling rig as the pipes go down into the earth. A fluid, for example drilling mud, is circulated through the drill string and up the annulus to remove cuttings from the hole and to lubricate the drill bit. Because of the number of drill pipes usually making up the drill string, and the extreme Wear conditions inherent in drilling operations, providing suitable conductors for transmission of electrical signals aldng the drill string has heretofore been a diflicult problem.

Various methods have been tried heretofore to transmit the electrical signals between downhole and the surface. The problems associated with providing. two, or even one, continuous insulated cables or electrical conduits along with the drill pipe for linking the downhole transducer with the surface transducer have led the art toward drill pipes with individual insulated electric conduits inside each pipe and some means for electrically connecting the conduits when the pipes are mechanically coupled. In this manner a circuit is formed with the drill string serving as one conductor and the electrically connected insulated conduits as the other conductor. Various methods of electrically connecting the conduits at the couplings of the drill pipes have been attempted. In eluded among these methods of electrically connecting the pipes are mating contact connections in the ends of each section of pipe. One of the most successful of the mating contact connectors is described in my copending application Serial No. 162,421. However problems have been encountered even in the most successful of the contact connectors because of current loss through the drill ing fluid to ground or shorting between the electrical cable connecting surfaces and the drill pipe at the drill pipe joints.

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The problems involved in electrical signal transmission along a segmented drill string in a borehole will be made clear by the following discussion. As fully presented and claimed in my copending application Serial No. 162,421, an electrical transmission system using the drill string as one conductor and electrically insulated conduits or cables secured interiorly of said drill string as a second conductor must have insulated electrical connectors to connect the conduits at the drill pipe joints. The electrical connectors electrically connect the insulated cables carried interiorly of each section of drill pipe when the drill pipes are mechanically coupled in a conventional manner by the drilling crew to form a drill string. Electrical connectors are provided in each end of a drill pipe. One of the electrical connectors in the drill pipe is adapted to make contact with a mating connector in another drill pipe when the two pipes are mechanically coupled together. The connectors must be provided with some type of electrical insulation to prevent electrical shorting between the drill pipes and connecting surfaces of the insulated cables.

Each mated connector sect-ion is protected from drilling fluid invasion to some extent by a fluid sealing means formed around the electrical contacting surfaces. However, it has been found that, under some conditions, as for example the extreme pressures encountered in well drilling, drilling fluid invasion of the electrical connectors occurs. When drilling fluid invasion occurs, some electrical current is lost to ground. This partial shorting reduces the signal voltage that is transmitted. When several of these shorts occur in the drill string electrical signals will be partially or completely prevented from being picked up.

It is a particular object of the present invention to improve electrical signal transmission in well drilling by increasing the leakage resistance to ground of electrically connected insulated conductors located in a fluid-filled well to prevent lossof current to ground by providing a directcurrent potential on the insulated conductors to cause an electrolysis through the drilling mud at the connecting joints of the conductors which results in the formation of an insulating gas film on the electrical contacts of the electrically connected insulated conductors.

Further objects and advantages of the present invention will become apparent from the following detailed description read in light of the accompanying drawings, which area part of this specification and in which:

FIG. 1 is a diagrammatic sectional view of an earth formation including a borehole and a drill string and showing a preferred embodiment of apparatus useful in practicing the method of the present invention.

FIG. 2 is a sectional view of an electrical connector provided in one end of a drill pipe.

FIG. 3 is a sectional view of a mating electrical connector in another end of a drill pipe.

FIG. 4 is a partial sectional view of coupled drill pipes showing mated electrical connectors.

FIG. 5 shows curves of variations of potential and applied current of a metal in an electrolytic solution.

FIG. 6 shows curves illustrating variation in leakage resistance with applied current.

FIG. 7 shows curves illustrating variation in leakage resistance with applied current.

FIG. 8 shows a curve illustrating how much directcurrent potential is needed to obtain the corresponding direct current of FIG. 6.

FIG. 9' shows a curve illustrating how much directcurrent potential is needed to obtain the corresponding direct current of FIG. 7.

Referring now specifically to FIG.- 1, an earth formation 20 is shown. Borehole 22 penetrates into the earth formation 20. A drill string 24 made up of drill pipes 26 extends into borehole 22. At the bottom of borehole 22 are drill bit 28 and drill collar 30. The drill string and bit are rotated in a conventional manner by rotary table 32. Extending interiorly up drill string 24 from electric signal generator or receiver 27 in drill collar 30 to the surface is an electrically insulated conductor such as cable 33. Insulated cable 33 has electrical connectors, represented generally as 34, at one end of each drill pipe and a mating electrical connector represented generally as 35 at the other end of each drill pipe.

A source of direct current iii which may be for example a direct-current generator is located on the surface near drill string 24. The negative terminal of source of direct current 40 is connected by suitable wiring to slip ring 41 and insulated cable 33. The positive terminal of source of direct current 40 is connected by suitable wiring to slip ring 42 and drill string 24. A circuit is completed by electrical contact at the transducer 27 between cable 33 and drill string 24 by means of wire 39. A directcurrent potential between the drill string 2 and cable 33 results when source of direct current 44 is activated. When a direct-current potential is applied to metals submerged in an electrolytic solution the electrical resistance of the system of metals and solution changes and gas bubbles are formed when the current density is sufliciently high. The present invention provides for increasing the resistance between the electrical connectors, for example mated connector 34 and connector 35, and the drill pipe or ground by bubble formation in the drilling fluid near the electrical contacts and by trapping the bubbles around the mated electrical contacting surfaces.

The electrical circuit on which the direct-current potential is applied also provides a circuit for transmission of an electrical signal to or from transducer 27 in drill collar 30 to transducer 50 on the surface. The signal transmitting circuit comprises transducer 27, electrically connected conduit 33, drill string 24, slip ring 42, slip ring 41, wire 45, wire 46 and transducer 50.

As known to those skilled in the art, transducer 27 may be adapted to send or receive a signal and may be positioned inside of drill collar 30. The transducer 27 may, for example, produce an electrical signal proportional to the force exerted by a drill bit 28. It is preferred that the transducer 27 produces or receives a frequency-modulated rather than an amplitude-modulated signal because of the difficulty in detecting difference in amplitude at the surface. In order to receive the signal at the surface, it is necessary that the leakage resistance between the mated electrical connectors 34 and 35 and the drill pipe string 24 or ground is high so that the signal will not be attenuated by shorting between the connectors and the drill pipe.

It is a particular feature of the present invention to improve the leakage resistance between the electrical connecting surfaces of mated electrical connectors, such as connector 34 and connector 35, and the drill string 24 by providing a direct-current bias between the drill string 24 and the electrically coupled cable 33 to cause electrolysis of the drilling fluid near the contacting surfaces of the electrical connectors and thus cause bubbles to be formed near the contacting surfaces of the electrical connectors thereby insulating the contacting surfaces of the electrical connectors. This insulation acts to increase the leakage resistance between the connectors and the drill string or increase the leakage resistance between the connectors and ground. The improvement in leakage resistance makes it possible to send a signal between transducers on the circuit composed of the drill string and the electrically coupled cable.

As shown in FIG. 1 an electric circuit is formed by the drill string 24 acting as one conductor and insulated electrically connected cable or conduit 33 providing the other conductor. It is also within the scope of the present invention to provide other means of forming the electrical circuit with drill string 24. For example, an

electrical insulating coating provided on the drill pipe and an electrical conducting coating provided over the electrical insulating coating will serve to complete an electric circuit with drill string 24 for signal transmission. Electrical connectors similar to connector 34 and connector 35 are provided at each end of the drill pipe in electrical contact with the conducting coating to electrically couple the coating as the drill pipes are coupled.

The electrical insulating coating is preferably a heatresistant metal oxide. A group of metal oxides including aluminum oxide and zirconium oxide have been developed. These oxides are heat insulators and electrical insulators and are the preferred insulating coating for the drill pipe. The coating must be made nonporous. Many processes well known in the art are available to coat a metallic surface such as a drill pipe with the oxides.

An electrical conducting coating is applied over the insulating coating. The preferred conducting coating for a particular application will depend on many alternatives such as the wear resistance to the particular drilling mud used in the operation and the particular insulating coating material used. It is preferred to use a conducting coating which may be applied in the field. Included among the group of useful electrical conducting coatings are titanium, iron or nickel coatings.

With reference to FIGS. 2 and 3 one embodiment of apparatus for electrically connecting electric cables secured inter-iorly of drill pipes when the pipes are mechanically coupled is shown. In FIG. 2 one end of a drill pipe, for example the male end, commonly known as the pin end is identified by the numeral 60. In FIG. 3 the mating end of a drill pipe, commonly called the box end is identified by the numeral 61.

According to the present invention the pin end 60 of a drill pipe, such as drill pipe 26, is provided with an electrical contact ring 62. Contact ring 62 is electrically connected to insulated cable 33. The contact ring 62 is mounted in a deformable mounting 63 such as rubber. The deformable mounting 63 electrically insulates the contact ring from pin end 60. The box end 61 of a drill pipe is shown in FIG. 3. The box end 61 and the pin end 60 are matable by means of the threaded connections. A contact ring 64 is held in box end 61 by mounting 65 Preferably mounting 65 is composed of material which does not deform as easily as deformable mounting 63. Contact ring 64 is electrically connected to an insulated cable 33. Contact ring 62 is adapted to make electrical contact with contact ring 64 when the pin end 60 is connected to box end 61.

Referring now to FIG. 4, a partial section of pin end 60 connected to box end 61 is shown. Contact ring 62 mates with contact ring 64 to make an electrical connection for cable 33 through the joint formed by box end 61 and pin end 60. The contact ring 62 in the deformable mounting 63 is depressed by the mating contact ring 64 in the more rigid mounting 65. The deformable mounting deforms and forms a fluid sealing means around the contacting surfaces of contact ring 62 and contact ring 64. The fluid sealing means helps to prevent drilling fluid invasion of the mated contact nings. Further, according to the present invention the fluid sealing means also acts to trap the gas formed near the electrical contacting surfaces. If drilling fluid does invade the mated contacts an electrical short is possible between the rings and the drill pipe.

It has been found that, due to the high pressures of the drilling mud, which may range upward of 10,000 p.s.i., mechanical means such as the fluid sealing means formed by the deformable mounting rings do not completely protect the contact rings from the drilling fluid. Therefore, current loss by shorting between the contact rings and the drill pipes will occur. Some means must be found to increase the leakage resistance of the flow path created by drilling fluid invasion. The path of possible current loss where the resistance must be increased is represented by R in FIG. 4.

It is a particular aspect of this invention to improve signal transmission in a fluid-filled borehole by establishing a direct-current potential across the drill pipe joints for a predetermined time to cause electrolysis of the drilling fluid near the electrical connections and to capture the resulting bubbles to insulate the electrical connecting surfaces of the insulated cables from the drill pipes to provide improved leakage resistance at electrically connected drill pipe joints When a direct-current potential is applied between two dissimilar metals such as a steel drill pipe 60 and the connecting rings 62 and 64 of FIG. 4 which might be copper or brass for example, that are partially submerged in a solution, the resistance of the circuit of metals and solution will depend upon the current density that is applied. Since the metal resistances are negligible, the over-all resistance can be divided into the resistances from the metal to the solution and the solution resistance. In the method of the present invention, we are concerned with both the contact resistance between metal and solution and the solution resistance.

Refer now to FIG. 5 where curves of the variation in potential and applied current are shown for copper in a 2% sodium chloride solution. The working electrode is iron. The curves reflect variations in potential and applied current for diiferent pH and temperature values. In all cases the curves indicate that when a small amount of current is applied, the resistance increases rapidly. This increase in resistance is reflected in the steep part of the voltage-current curve. At higher applied currents the voltage increase is much less and the resistance decreases. The transition from the part of the curve where potential changes greatly with ap plied current to the part of the curve where potential changes slightly with applied current is usually associated with the beginning of the release of gas from the metal. The shape of the curve i-srelated to the efliciency with which gas is swept from the metal surface. For example, the resistance of a given polarization curve will decrease much more rapidly if mechanical agitation removes the bubbles from the metal surfaces as they are formed. Conversely if the bubbles are trapped near or on the metal surfaces, the resistance will increase.

It is a particular aspect of the present invention, then, to apply a direct-current potential across the electrical connections of connected drill pipes located in the drilling fluid in a well to producea gas and to trap said gas between the electrical contacting surfaces and the drill pipe during the transmission of an electrical signal between the surface and downhole or downhole and the surface to improve the leakage resistance between the electrical contacting surfaces and the drill pipes and to thus prevent attenuation of the signal due to current loss at the connecting joints. Since the number of connections in a drill string will vary depending on the depth of the borehole and since the electricalcharacteristics of the contacts will vary with configuration, the level of direct-current potential for any particular application may vary. However the direct-current potential applied to the drill string must be at a suflicient level to produce electrolysis at each of the joints in the drill string where drilling fluid invasion of the electrical contacts has occurred. The curves in FIG. 5 will aid in determining the minimum direct-current potential.

The results of demonstrations conducted by the method of the present invention are graphically shown in FIG. 6. FIG. 6 shows two curves of resistance versus applied current. The resistance is equivalent to the leakage resistance (R between the electrical connecting surfaces and the steel drill pipe. In the actual demonstrations electrical connectors similar to the connectors shown in FIGS. 2 and 3 were coupled together. In the case of Curve 1 the pin end and the box end were coupled in a manner so that around the contacting surface to form a sealing means to trap the gas bubbles around the contacting surface. The deformation of this mounting was about 0.02 inch.

To provide a basis for comparison and to show the improvement in leakage resistance of the present method, the leakage resistance was first tested with connectors such as in FIG. 2 and FIG. 3 without the application of a direct-current potential. The pin end and box end containing the connectors were connected in a manner to provide for about an 0.02 inch deformation of the rubber ring. The connectors were submerged in a drilling fluid with a pH of 10.0 and at a temperature of 21 C. The leakage resistance to ground (R was ohms.

With particular reference to Curve 1 of FIG. 6 the improvement in leakage resistance provided by the present invention is shown. The pin end and box end containing the connectors were connected in the same manner as above to provide an 0.02 inch deformation of the rubber mounting. A direct-current potential was developed between the electrical contacting surfaces and the steel pipe. As shown in Curve 1 of FIG. 6 the leakage resistance increased from 120 ohms to more than 1400 ohms at a current of 1 milliamp. The leakage resistance decreased as the applied current was increased. However, the leakage resistance leveled oif at about 500 ohms at higher applied currents. With the improved leakage resistance provided by the present invention much less current is lost by shorting and consequently an improved signal is received.

Curve 2 of FIG. 6 shows the leakage resistance with an open cell in the drilling mud. In other Words only the box end of the drill pipe containing a contacting ring was placed in the drilling fluid. Curve 2 shows that the leakage resistance in this case also increases to about 1400 ohms at 1 milliamp. However at higher currents the gas formed was not trapped around the contact ring and the leakage resistance decreased to a low value. This clearly demonstrates the value of trapping the bubbles around the electrical contacting surfaces and retaining the bubbles around the contacting surfaces during signal transmission.

With reference to FIG. 7, a similar demonstration to the one described above was performed in which the drilling fluid was replaced by 2% sodium chloride solution. This was done to demonstrate the eflfectiveness of the method of the present invention in a highly electrolytic solution. As is shown in Curve 3 of FIG. 7, a leakage resistance of several hundred ohms is maintained when a direct-current potential is applied. This leakage resistance is maintained by providing a fluid sealing means to trap the gas film around the electrical contacts. The fluid sealing means was formed by deforming the rubber mounting 0.02 inch. Curve 4 of FIG. 7 illustrates the resistance of an open cell and points up the desirability of trapping a the bubbles between the connecting surfaces and the drill pipe when the applied current exceeds a few milliamps.

FIG. 8 shows the direct-current potential needed to obtain the corresponding direct-current value shown in Curve 1 of FIG. 6. FIG. 9 shows the direct-current potential needed to obtain the corresponding direct-current value shown in Curve 3 of FIG. 7. The level of directcurrent potential to establish on the drill string circuit depends on a number of factors. The number of drill pipes in the drill string and thus the number of electrical connections is an important factor. The particular drilling fluid is important in establishing the level of direct-current potential. The configuration of the electrical connectors in each section of drill pipe will also affect the amount of direct current potential required.

The increase in resistance to short-ing between the electrical connecting surfaces and the drill pipe does not occur simultaneously with the application of the directcurrent potential. Some lapse of time after the directcurrent potential is established is required to increase the leakage resistance from a relatively low value to a high value. A demonstration was conducted to show the time lag from the application of the direct-current potential at the coupling to the increase in resistance. The demonstration was conducted in a 2% NaCl simulated drilling fluid in a manner similar to the demonstrations described above. The demonstration showed that about /2 second of directcurrent potential application is required to increase the leakage resistance to a maximum value. When the resistance is to be improved in more than one coupling the improvement occurs in a step-like process. That is, one coupling will be polarized first. This may take, for example, /2 second. Then another coupling in the drill string will be polarized. The time required to polarize the second coupling is in addition to the time required for the first.

The time required to improve the leakage resistance per coupling is important because in drilling deep wells as much as 15,000 feet of drill pipe and 500 or more couplings may be involved. Since each coupling will be polarized in a step-like process, the direct-current potential must be applied to the drill string for a substantial time prior to sending an information-bearing signal. In many drilling operations the direct-current potential should be applied to the drill string at least 1 to 2 minutes prior to sending an information-bearing signal.

In a broad aspect then the present invention comprises improving transmission of an information-bearing signal in a fluid-filled borehole with the drill string serving as one conductor and electrically connected insulated cables serving as the other by the steps of applying a directcurrent bias on the drill string and the insulated cables for a predetermined time prior to sending said signal on said drill string and said insulated cables to produce a gas film at the electrical connections of said insulated cables and retaining said gas film around said electrical connections during transmission of said signal.

Depending on a number of factors it may be necessary not only to apply the direct-current potential to the drill string to increase leakage resistance at the connections to a maximum prior to the transmitting of an informalion-bearing signal but also to continue to apply the direct-current bias during the transmission of the signal to maintain the maximum leakage resistance at the electrical connections of the drill string. It is also Within the scope of this invention to apply the direct-current potential prior to the transmission of the signal and not during actual transmission of the signal if the gas film will remain trapped long enough around the electrical connections to allow the signal to be transmitted. However, since the application of a direct-current bias to the drill string circuit is readily compatible with sending an alternating current signal, it is well within the skill of the art to supcrimpose one on the other. For example, a requency modulated alternating current signal is readily superimposed over a direct-current potential.

While the method of this invention has been described with an embodiment of apparatus which includes the drill string as one of the conductors and a segmented insulated conductor as the other for transmitting a signal in a well, it is obvious to those skilled in the art that the method of the invention is also applicable in situations where a pair of segmented insulated conductors having electrical connections in the drilling fluid are utilized to transmit a signal in a fluid-filled borehole. The invention having been fully described and illustrated, I claim:

1. In a method of electrically transmitting a modulated alternating current signal in a fluid-filled borehole in which coupled drill pipes form a first electrical conductor and insulated cables which are electrically connected at the drill pipe couplings form a second electrical conductor, the improvement comprising applying a directcurrent potential to the conductors for a predetermined time prior to transmitting said modulated alternating current signal on said mnductors.

2. In a method of electrically transmitting a signal in a fluid-filled borehole in which coupled drill pipes form a first conductor and insulated cables which are electrically connected at the drill pipe joint form a second conductor, the improvement comprising esbablishing a directcurrcnt potential on the conductors prior to sending said signal to produce a gas film on the metal surfaces which electrically connect said insulated cables and trapping said gas film around said surfaces while sending said signal on said conductors.

3. The method of claim 2 where the direct-current potential is applied during transmission of the signal.

4. In a method of electrically transmitting a signal between transducers at least one of said transducers being located in a fluid-filled borehole, said transducers being electrically connected by coupled drill pipes as one conductor and by insulated cables which are electrically connected at the drill pipe joints as the second conductor, the improvement comprising establishing a direct-current potential on the conductors for a predetermined time prior to sending said signal to cause gas bubbles to be formed on the connecting surfaces of said cables, trapping said gas bubbles between said connecting surfaces and said drill pipes and transmitting said signal on said conductors.

5. A method of electrically transmitting a signal in a fluid-filled borehole comprising forming a first electrical conductor in said borehole, said first electrical conductor comprising a plurality'of coupled drill pipes, forming a second electrical conductor in said borehole, said second electrical conductor comprising a plurality of insulated cables having electrical connections at the joints of said drill pipes, establishing a direct-current potential on the conductors to produce a gas film on the electrical connections of said insulated cables and the drill pipes by the electrolysis of the borehole fluid, trapping said gas film around said connections, producing an electrical signal and transmitting said signal over said conductors while maintaining said gas film around said connections.

6. Apparatus for electrically transmitting an alternating current signal in a fluid-containing borehole comprising means producing an alternating current signal, means for receiving said produced signal, a plurality of coupled drill pipes as :a first conductor between said signal-producing means and said signal-receiving means, a plurality of electrically insulated cables having electrical connections at the couplings of said drill pipes as a second conductor between said signal-producing means and said signal-receiving means, and means for establishing a direct-current potential on said first conductor and said second conductor concurrently with the sending of said signal.

7. Appaartus for electrically transmitting a signal in a fluid-containing borehole comprising means producing an electrical signal, means for receiving said produced signal, a plurality of coupled drill pipes as a first conductor between said signal-producing means and said signal-receiving means, a plurality of electrically insulated cables having electrical connections at the couplings of said drill pipes as a second conductor between said signal-producing means and said signal-receiving means, means for establishing a direct-current potential on said first conductor and said second conductor to cause a gas film to be formed on said electrical connections, and

means for trapping said gas film around said electrical connections.

8. Apparatus for electrically transmitting a modulated alternating current signal in a fluid-containing borehole comprising means producing an alternating current signal, means for receiving said produced signal, a plurality of coupled drill pipes as a first conductor between said signal-producing means and said signal-receiving means, an electrically insulated conduit in each of said drill pipes, means electrically connecting adjoining electrically insulated conduits at each joint of said drill pipes so that the electrically insulated conduits form a second conductor between said signal-producing means and said signal-rei electrically insulated conduits at each joint of said drill pipes so that the electrically insulated conduits form a second conductor between said signal-producing means and said signal-receiving means, means for establishing a direct-current potential on said first conductor and said second conductor to cause a gas film to be formed on the electrical connections of said insulated conduits, and

means trapping said gas film between said connections and said drill pipes.

10. The apparatus of claim 9 where the electrically insulated conduit in the drill pipes comprises a coating of electrical insulating material on said pipe and a coating of electrical conducting material over said electrical insulating material.

11. In a method of electrically transmitting a signal between transducers, at least one of said transducers being located in a fluid-filled borehole, said transducers being electrically connected by coupled drill pipes as one conductor and by insulated cables which are electrically connected at the drill pipe joints as the second conductor, the

improvement comprising establishing "a direct-current potential onthe conductors for a predetermined time prior to sending said signal to cause gas bubbles to be formed on It) the connecting surfaces of said cables, trapping said gas bubbles between said connecting surfaces and said drill pipes and maintaining said direct-current potential on the conductors while transmitting an alternating current sig nal on said conductors.

12. In a method of'electrically transmitting a signal between transducers, at least one of said transducers being located in a fiuid filled borehole, said transducers being electrically connected by at least a pair of segmented insulated conductors, said conductors having electrical connections in the drilling fiuid, the improvement comprising establishing a direct-current potential on the conductors for a predetermined time prior to sending said signal to cause gas bubbles to be formed on the connecting surfaces of said conductors, trapping said gas bubbles between said conductors and transmitting a signal on said conductors.

13. In a method of electrically transmitting a signal on a pair of connected conductors located in a well containing an electrolytic drilling mud, the improvement comprising establishing a direct current potential on the conductors to cause electrolysis of the drilling mud adjacent the connections of said conductors and the formation of a gaseous film therebetween, and maintaining said direct .current potential on the conductors while establishing an alternating current potential on said conductors for the transmission of a signal thereon.

14. The method of claim 13 further characterized by trapping the gaseous film between the connections of the conductors.

References Cited in the file of this patent UNITED STATES PATENTS 

13. IN A METHOD OF ELECTRICALLY TRANSMITTING A SIGNAL ON A PAIR OF CONNECTED CONDUCTORS LOCATED IN A WELL CONTAINING AN ELECTROLYTIC DRILLING MUD, THE IMPROVEMENT COMPRISING ESTABLISHING A DIRECT CURRENT POTENTIAL ON THE CONDUCTORS TO CAUSE ELECTROLYSIS OF THE DRILLING MUD ADJACENT THE CONNECTIONS OF SAID CONDUCTORS AND THE FORMATION OF A GASEOUS FILM THEREBETWEEN, AND MAINTAINING SAID DIRECT CURRENT POTENTIAL ON THE CONDUCTORS WHILE ESTABLISHING AN ALTERNATING CURRENT POTENTIAL ON SAID CONDUCTORS FOR THE TRANSMISSION OF A SIGNAL THEREON. 